How to create a flat base to a photogrammetry model in Blender?
What I do usually is profit from the new vertex group options for generated geometry in the solidify modifier from version 2.83+.
First let's say we have an ocean modifier on an object.

Create a new vertex group.

Add a solidify modifier, set the Shell Vertex Group to your new group, and the Thickness to 0. This will effectively duplicate your geometry in place.

Now add a plane at the altitude at which you want the bottom of your foundation to be. Scale it so it is larger than your terrain.

Add a shrinkwrap, set the target to your plane, vertex group to your new vertex group, set the mode to project on the Negative Z axis. Hide your plane.

Now add a new material for the dirt/soil, assuming the first one is your photogrammetry raster info.

Get back to the solidify modifier and set the material offset to 1. Result :

With &quot;proper&quot; shaders:

Note : The geometry will be ugly if viewed from upside-down but what you can do is check &quot;Only rim&quot; in the solifidy modifier to only keep the rim of the object.